Accelerated testing of computer chips for “soft” fails can be accomplished by exposing the chips to a beam of protons, neutrons, or other ions. The high energy protons and neutrons can cause a “spallation” reaction to occur when they collide with nuclei in the chip, for example copper, oxygen, or silicon. (See Henry H. K. Tang, and Kenneth P. Rodbell, MRS Bulletin, Volume 28, No. 2, February 2003, 111.) This primary reaction then causes the release of heavily-ionizing particles which may hit a sensitive region and cause a soft fail. Alternatively, if the heavy ions can deposit enough charge, they can cause a fail as well. A tester is connected to the chip and can load specific patterns onto, and then read the pattern from, the chip after the exposure to the external beams. (See J. F. Ziegler, H. P. Muhlfeld, C. J. Montrose, H. W. Curtis, T. J. O'Gorman, and J. M. Ross, IBM Journal of Research and Development, Vol. 40 No. 1 Jan. 1996 (51).)
Most cosmic rays near the earth's surface include neutrons, and at higher elevations, a mixture of neutrons, protons, muons and pions. (See J. F. Ziegler, IBM Journal or Research and Development, Vol. 40 No. 1 Jan. 1996 (19).) It is believed that the soft error rate (SER) for chips caused by cosmic ray neutrons in the ˜50 MeV and above energy range is nearly the same as that caused by protons at the same energy. (See Joint Electron Device Engineering Council, (JEDEC) Standard JESD89, Measurement and Reporting of Alpha Particle and Terrestrial Cosmic Ray-Induced Soft Errors in Semiconductor Devices.) Since proton beams are more readily available than neutron beams at high energy, chip manufacturers and vendors generally perform the high-energy portion of the cosmic-component of accelerated SER testing using proton beams. It is desirable to perform neutron-induced SER tests at low energy (˜14 MeV) since the proton and neutron-induced SER may differ at these low energies.
Several laboratories that have SER programs for proton and neutron testing are listed in annex D of JEDEC, supra.
The overall soft error rate caused by terrestrial cosmic rays is found by integrating the product of the empirically-determined SER cross section (measured at several energies) and the cosmic ray neutron flux, over energy. (See JEDEC, supra.) The accelerators used to generate the proton beams are generally cyclotrons or linear accelerators with somewhat complicated beam transport systems. It has generally been found to be time consuming to reduce the energy of these beams since many ion-optical lenses and steering magnets need to be retuned. Beam energies are lowered by inserting a “degrader” into the path of the proton beam. (See Ziegler and Muhlfeld et al., supra.) The primary beam loses energy and spreads out laterally as it traverses through a block of Plexiglas or Lucite. This degraded beam impinges on the computer chip at lower energy than the primary beam, with a larger energy spread and larger lateral dimensions.
Heretofore, the degraders have been changed by hand. To avoid personal exposure to severe radiation, the beam of protons has been either interrupted upstream of the degraders by insertion of a beam stop or Faraday cup, or the accelerator has been turned off, and the degraders were swapped by hand. The degraders, themselves, become radioactive, emitting β's and γ's after being irradiated by the proton beam, thus presenting potential safety problems. Accordingly, a need has been recognized in connection with overcoming these problems, among others.